1. Field of the Invention
The present invention relates to a gas venting arrangement incorporated into a mold for use in a die casting machine or other molding machine, more particularly to an improvement of the gas venting arrangement of the axial melt impinging type, disclosed in Australian Pat. No. 516,938 and U.S. Pat. No. 4,431,047 issued Feb. 14, 1984.
2. Description of the Prior Art
The die casting method is widely used for manufacturing large quantities of precision products. Since molten metal is charged at a high speed under a high pressure into the mold cavity, however, gases are not sufficiently vented from the mold cavity, resulting in voids in the final product. This method therefore is often unsuitable for preparing high quality products for which the absence of voids is required.
To overcome this disadvantage, an improved gas venting arrangement as disclosed in the above-mentioned patents was invented. In this arrangement, a large quantity of gas can be easily and reliably vented irrespective of the kind of cast product or mold, and a high quality die cast product free of entrained gas can be obtained.
According to the disclosed arrangement, when the melt is injected, a discharge passage extending from the cavity of the mold to the outside of the mold is opened by valve action. When the gas in the cavity is completely discharged through the discharge passage, the inertia force of the injected melt, which has advanced from the interior of the cavity, is directly imposed on the valve, that is, the melt strikes the valve. This assuredly and promptly moves and closes the valve to shut the discharge passage and prevent flow-out of the melt through the discharge passage.
It was found, however, that, when the melt which is supposed to strike the valve flows discontinuously, it does not always close the valve completely and assuredly. When the leading portion of the discontinuous melt initially strikes the valve, it may move the valve upward against the downward force of a spring provided in the arrangement to change the valve from the opened position to the closed position. After the leading portion, however, the valve is returned to the opened position by the downward force of the spring until the following portion of the melt reaches the valve.
Under such circumstances, when the following portion of the melt approaches the valve, the leading portion is already solidifying at the front face of the valve and adhering to the valve face and inner wall of the discharge passage near the valve. As a result, the impinging force of the following portion of the melt against the valve is considerably reduced. This prevents the valve from returning smoothly to the closed position and results in incomplete valve closure and axial valve oscillation during the discontinuous impingement.
The first mentioned gas venting arrangement was improved to overcome the above defects and patent applications were filed for this improvement in Japan, the U.S. (Ser. No. 322,264 filed Nov. 17, 1981 and now U.S. Pat. No. 4,489,771 issued Dec. 25, 1984), and several other countries. As disclosed in the Japanese unexamined publication (No. 57-88962) of the application, the improved invention provides the following gas venting arrangement incorporated into a mold formed of stationary and movable mold halves together defining a cavity to be filled with a melt. The arrangement comprises:
(a) a valve chamber including an enlarged forward portion formed in the mold and a constricted rear portion formed in a member separate from the mold, and a valve seat formed between the forward and rear portions;
(b) a gas vent passage formed in the mold connected to the cavity and to the forward end of the forward portion of the valve chamber;
(c) a gas discharge passage formed in the mold and opening on an inner side surface of the rear portion of the valve chamber to communicate with the outside of the mold;
(d) at least one by-pass passage formed in the mold branching from a point on the gas vent passage to an opening on an inner side surface of the forward portion of the valve chamber;
(e) a valve slidably received in the forward portion of the valve chamber for movement between a first position, wherein the valve cooperates with the valve chamber to prevent the gas vent passage from communicating with the gas discharge passage through the forward portion of the valve chamber and to permit the by-pass passage to communicate therewith through the valve chamber, and a second position, wherein the valve rests against the valve seat and cooperates with the valve chamber to prevent both the by-pass passage and the gas vent passage from communicating with the gas discharge passage through the valve chamber;
(f) first means for biasing the valve into the first position and permitting movement of the valve into the second position under axial impingement against the valve of a portion of the melt injected into the cavity and forced to flow through the gas vent passage into the forward portion of the valve chamber, the first biasing means being adjusted and the by-pass passage being dimensioned and configured to permit movement of the valve into the second position before a second portion of the melt reaches the forward portion of the valve chamber through the by-pass passage;
(g) second means for biasing the valve into the second position after the valve is forced to move from the first position to the second position by an initial impingement of the melt; and
(h) means for releasing the valve from the second biasing means to return from the second position to the first position.
The embodiment of the above arrangement as disclosed comprises a spool forming the rear portion of the valve chamber and a piston-cylinder apparatus. The spool is connected to the piston, so that it can be moved in the axial direction of the valve extension by actuation of the piston. The valve extension is connected to the spool by means of a spring and thus it is forced to move rearward according to the rearward movement of the spool. The above-mentioned releasing means is a stopper mechanism for stopping the rearward movement of the valve relative to the spool while the spool is forced to move rearward. The above-mentioned first biasing means is a retention mechanism for retaining the valve in the first position when the stopper mechanism functions. The stopper mechanism includes opposite arms radially extending from the valve extension and stopping means provided on the spool or another member for the arms. The retention mechanism includes an elastic or spring member to be engaged with a specific portion of the valve extension.
Such an embodiment, however, is complicated, and the height of the arrangement is inevitably large.
Further, as the mold is cold at the start of injection of the melt, the melt tends to solidify before it arrives at corners of the mold cavity, resulting in an incomplete molded product. Therefore, several shots of melt are normally injected at a low speed to warm the mold before ordinary or normal injection is carried out. If the above arrangement is directly applied to a process in which the injection speed is low in the initial stage and is only later increased, there is the disadvantage that the weak inertia force of the initial melt will not close the valve enough, creating the risk of intrusion of the melt into the valve chamber. If the melt intrudes into the valve chamber, it will solidify there and obstruct the gas venting arrangement, necessitating repair or replacement of the arrangement. Repair or replacement, of course, will force a stop of the molding operation and reduce productivity.
Now, the previous arrangement was designed so that, when the mold is opened, the entire spool, that is, the removable rear portion of the valve chamber, is raised; the retreating movement of the valve is stopped midway of the retreating or rearward movement of the spool; and the valve is biased into the first or opened position by the stopper mechanism and so that, when the mold is closed, the spool is brought down to a predetermined position with the valve open in the first position. At the start of actuation of the piston, however, the pressure of the hydraulic fluid acts abruptly and creates a shock. As a result, a force is imposed on the valve exceeding the retention capacity of the mechanism for retaining the valve in the first position, thus forcing the valve into the second position. This prevents the gas venting arrangement from functioning.